The present invention relates generally to soldering substrates, and more specifically to applying solder to the underside of leaded substrates.
Often, printed circuit board structures are soldered by automated techniques such as wave soldering. Solder bridging between leads is a problem in these processes. U.S. Pat. No. 4,339,784 addresses this problem when soldering leads to a circuit board by providing foil pads to draw off excess solder to avoid bridging between leads. However, this process does not allow selective placement of solder on the substrate to avoid coating leads with solder. Oil has been used to avoid bridging but requires post-soldering clean-up and does not prevent soldering of the leads.
U.S. Pat. No. 4,304,640 shows the use of a mask to selectively solder the surface of a circuit board. In this process, first a continuous conductive surface is formed on a board. Next, a mask is formed on the surface to leave predetermined exposed areas. Solder is electroplated onto these areas, and the mask and remaining conductive surface are removed. This patent uses a leadless substrate and, thus, does not address problem of solder bridging between leads or the prevention of solder coating leads. Masking leads would be a difficult and time-consuming step. Also, exact alignment of the mask is required because only the unmasked areas will be soldered.
U.S. Pat. No. 4,332,341 shows the bonding of solid solder preforms to selected areas of a substrate before melting. Selective placement is accomplished with a vacuum applicator and transfer occurs with pressure and heat. Exact alignment is necessary because the solder is transferred at a temperature below its melting point, therefore it will not self-align on transfer. In addition, the apparatus could not be used on the face of a substrate with leads as the vacuum applicator is flat and covers the entire substrate. The problem with the present soldering methods is soldering the underside of a substrate having leads without soldering the leads.
Thus, it is an object of the present invention to provide a process for soldering the underside of a substrate having leads.
It is another object of the present invention to provide a process for soldering in between the leads of a substrate without soldering across the leads.
It is another object of present invention to provide a process for selectively soldering areas of a substrate in between the leads of a substrate while leaving other areas free of solder.
These objects are achieved in accordance with a preferred embodiment of the present invention by a method using a solder carrier having a solder pattern adhered thereto by heat to solder the plated areas of a substrate. Solder paste on one side of a solder carrier is applied to a substrate with that solder side facing the substrate. The substrate, solder and solder carrier are heated to transfer the solder to the plated areas of the substrate at a temperature sufficient to allow self-alignment.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.